1. Field of the Invention
The present invention relates to a wireless power and data transmission system for transmitting electric power and data, a power transmitting apparatus, and a power receiving apparatus.
2. Description of the Related Art
It has been attempted to transfer electric power necessary to operate an electric device wirelessly or in a non-contact manner (contactlessly). For example, in the fields of electric shavers, electric tooth brushes and the like, products adopting a wireless power transfer method using the principle of electromagnetic induction described in Japanese Laid-Open Patent Publication No. 2008-206233 (hereinafter, such a wireless power transmission method will be referred to as an “electromagnetic induction method”) have been put on the market and have successfully improved the convenience for users.
Recently, a wireless power transmission method using the resonance phenomenon in an electromagnetic field are known (e.g., U.S. Laid-Open Patent Publication No. 2010/0141042). This wireless power transmission method has advantages that the transfer distance is extended and that the degree of freedom in the relative locations of a power transmitting device and a power receiving device can be improved as compared with the electromagnetic induction method. This wireless power transmission technology also referred to as the resonant magnetic coupling method has been currently on research and development, and wireless power transmission is expected to be applied in a wider range of fields.
According to the resonance magnetic coupling method, a power transmitting device and a power receiving device each include a resonator. Only when the resonance frequency of the resonator in the power transmitting device (transmitting-side resonator) and the resonance frequency of the resonator in the power receiving device (receiving-side resonator) match each other, power is transferred from the transmitting side to the receiving side by use of the resonance phenomenon.
In general, the efficiency of the wireless power transfer is in proportion to kQ, which is the product of the coupling coefficient k between the transmitting-side resonator and the receiving-side resonator and the Q factor (Quality Factor) of the resonators. Generally for the resonant magnetic coupling method, resonators having the Q factor in the order of several thousands are used. Owing to this, even when the coupling coefficient k is small because the power transmitting device and the power receiving device are away from each other by a long distance or because the axis of the coil of the transmitting-side resonator and the axis of the coil of the receiving-side resonator are shifted from each other, a high power transfer efficiency can be achieved.
According to the resonant magnetic coupling method, as disclosed in Takehiro IMURA et al., “Proposal of Wireless Power Transfer via Magnetic Resonant Coupling in kHz-MHz-GHz”, The 2010 Proceedings of The Institute of Electronics, Information and Communication Engineers, BS-9-5, March 2010, the resonance frequency of the resonators can be set to any frequency. It is now assumed that the resonators are designed by use of a frequency in the band of several tens of megahertz as the resonance frequency, such that the 3 dB bandwidth of the frequency characteristic of the resonators is several hundred kilohertz to several megahertz. In this case, data communication can be made in which a magnetic carrier wave for power transfer is modulated with data having a transfer rate of several hundred kilobits per second to several megabits per second. In this manner, overlay transfer of power and data can be realized. Hereinafter, such transmission will be referred to as “wireless power and data transmission”.
An example of data transferred by means modulation of a magnetic carrier wave is an audio signal. An audio signal is in a band of 20 Hz to 20 kHz, and therefore can be sufficiently transferred by wireless power and data transmission by the resonant magnetic coupling method using a band of several tens of megahertz. Japanese Laid-Open Patent Publication No. 2009-153056 discloses an example of wireless power and data transmission of an audio signal. A system disclosed in Japanese Laid-Open Patent Publication No. 2009-153056 includes a power transmitting apparatus for performing AM (Amplitude Modulation) modulation on a magnetic carrier wave used for wireless power transfer and transferring the resultant magnetic carrier wave, and a power receiving apparatus for receiving an audio signal having a sufficiently level of power to drive a speaker from the magnetic carrier wave transmitted from the power transmitting apparatus. By use of the power transmitting apparatus and the power receiving apparatus disclosed in Japanese Laid-Open Patent Publication No. 2009-153056, a wireless speaker apparatus which does not need a power supply such as a batter or the like is realized.
According to the wireless power transfer system disclosed in Japanese Laid-Open Patent Publication No. 2008-206233, data which authenticates that the power transmitting device and the power receiving device are both proper devices is transferred together with the power. Owing to such a structure, power transmission to an unauthorized power receiving device, to a power receiving device which does not need power, an item other than a receiving device or the like can be avoided. Therefore, the safety of wireless power transfer is improved.
Japanese Laid-Open Patent Publication No. 2008-206305 discloses a power transmitting apparatus for transmitting 0 and 1 of binary data as being distinguished from each other by using different frequencies for the magnetic carrier wave for 0 and 1. The frequency characteristics of resonators are frequency-selective with the resonance frequency fo being the peak. Therefore, when the frequency of a magnetic carrier wave is changed, the amplitude of the magnetic carrier wave is also changed. A power receiving apparatus disclosed in Japanese Laid-Open Patent Publication No. 2008-206305 can transmit 0 and 1 of binary data to the power transmitting apparatus as being distinguished from each other by use of an amplitude change of the magnetic carrier wave which is caused by a switch on the load of the power receiving circuit (load modulation).
In order to realize the above-described wireless power and data transmission, data such as an audio signal, control information or the like needs to be transferred by use of a magnetic carrier wave for transmitting power. Now, a case where, as in the apparatus described in Japanese Laid-Open Patent Publication No. 2009-153056, a magnetic carrier wave for power transfer is AM-modulated with an audio signal will be discussed. When the magnetic carrier wave which has been AM-modulated is further modulated with data such as a control signal or the like by the power transmitting device, the amplitude of the magnetic carrier wave is changed. As a result, a problem occurs that the sound reproduced by the receiving-side speaker is changed in the volume, is distorted, or includes noise.
This problem is not limited to a system for transmitting an audio signal to a speaker, and occurs in substantially the same manner in any system for transmitting data as being overlaid on power. Namely, there is a problem that when the magnetic carrier wave is modulated with data to be transmitted, the amplitude of the power received by the power receiving device is changed, which destabilizes the power transfer.